1. Field of the Invention
This invention relates to a method for producing an anisotropic rare earth magnet, and in particular to a method for extruding a compacted material in order to obtain a better yield of the anisotropic rare earth magnet excellent in magnetic properties.
2. Description of the Prior Art
Rare earth magnets represented by R-Fe-B (R is shown on behalf rare earth metals of lanthanum series) are provided in two types as mentioned hereunder;
(a) a sintered magnet which is made into an anisotropic magnet through a process of casting the molton base alloy into an ingot, pulverizing the ingot into super fine powder, pressing the powder in a magnetic field and sintering it, and
(b) a super-quenched magnet which is given with magnetic anisotropy through a process of making a thin flake by cooling the molton base alloy super-rapidly, molding a compacted material with magnetic isotropy by compressing coarse grained powder of the thin flake of the base alloy and deforming the compacted material plastically.
The anisotropic rare earth magnets obtained through the aforementioned processes have excellent magnetic properties, therefore it is very useful to apply these magnets to small-sized electric motors used for various automatizing apparatuses in order to make the motors lighter and smaller.
Although it is desirable to make the magnet in a ring shape given with magnetic anisotropy in the radial direction in order to apply the anisotropy rare earth magnets to the motors, there is a problem since it is difficult to give a magnet field in the radial direction at the time of molding the powder in a magnetic field in the case of the aforementioned sintered magnet.
In the case of the super-quenched magnet, it is possible to give the magnetic anisotropy in the utmost limit even for the ring-shaped magnet because the magnetic anisotropy is given by the plastic deformation without forming in the magnetic field.
As a method for giving the magnetic anisotropy by the plastic deformation, heretofore, it is taken to extrude the compact material with magnetic isotropy formed in a hollow or solid circular plate-like shape, as disclosed in U.S. Pat. No. 4,963,320, for example.
An example of the extruding is shown in FIG. 4. In the figure, numeral 100 is a cylindrical die formed in a thick-walled cylindrical shape, numeral 102 is a bottom die forming a bottom part of a mold.
Numeral 104 is a core punch and numeral 106 is a sleeve punch disposed movably in a molding cavity 108 formed between the core punch 104 and the cylindrical die 100. The mold is constructed from the cylindrical die 100, the bottom die 102, the core punch 104 and the sleeve punch 106.
The bottom die 102 is provided with a hollow part 112 to receive a slender part 110 of the core punch 104.
In this method for giving anisotropy, a hollow circular plate-like (ring) shaped compacted material 114 is charged into the cylindrical die 100 of the mold, subsequently the compacted material 114 is extruded backwardly by pressing the core punch 104 into the compacted material 114 at the same time of compressing a free surface of the compacted material 114 fronting on the molding cavity 108 with the sleeve punch 106 moving back according as the progress of the extruding, thereby making the compacted material 114 anisotropic in the radial direction at the same time of forming the compacted material 114 into a hollow cylindrical magnet material.
However, in the aforementioned extruding method, magnetic properties at the upper end portion of the cylindrical magnet material shown with symbol A in FIG. 4B are not so good as compared with, for example, a portion shown with symbol B in this figure, and there is a problem since it is not possible to use the upper end portion A practically.